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> What they don't mention is how efficient the solar cells produced are.

This... may or may not matter. When it comes to solar power on the Moon, there are significant advantages and some serious disadvantages. The most obvious advantage is that there is no weather and no atmosphere. This means a lunar solar cell has way more potential power generation than an terrestrial solar cell. The big disadvantage is the long day night cycle. The Moon is tidally locked with Earth with an orbit of 28 days so that's basically 14 days of day and 14 days of night.

You have to work around this problem with batteries, which add a ton more mass, having panels (and settlements most likely) at the poles or running a network of solar cells around the Moon with power cables.

Settling the poles makes sense as this is where the most water is.

But having a network of panels probably makes a lot of sense too. For one, aluminium is abundant so making electrical cables should be entirely possible.

> Another big question is how much mass needs to be shipped up from earth to manufacturer solar cells.

Yes, this is a huge factor, probably way more than efficiency is. Or at least we should look at power generation per unit mass shipped from EArth.

If they're going to make solar panels on the moon, perhaps the creativity won't stop there: Maybe we don't need to ship batteries, either.

They can make batteries out of moon rocks [1], use fly wheels, melt lava, etc. None super efficiently, but as a start.

[1] https://en.wikipedia.org/wiki/Gravity_battery

You have to work around this problem with batteries...

It's only a problem if you want to use the energy for systems that must be always available. If you intend to use it to power a factory, let's say to make more panels, you just operate 50% of time that, without clouds or any atmosphere filter, is actually better than on Earth.

For continuous support, take a nuclear reactor or build a wired net near a pole so when night comes to one station, you receive power from another.

> We do have materials which can withstand this environment, but how long will they last? How much power can the whole set up produce and would it be more than landing the setup's weight in solar cells?

Oh, of course this is terrible now. It would have a very, very large required scale to come out ahead compared to just shipping power or power generation some other way (if this is true at any scale). It is also surely not fully baked yet.

On the other hand... it's a notable step towards being able to build "big" in space and might be worth doing to learn more about the practical aspect even if it's higher risk and more costly than shipping a bunch of PV.

The bigger problem is how practical is it to use PV at all on much of the moon.

>The bigger problem is how practical is it to use PV at all on much of the moon.

I think part of the solution will involve using "regular stuff" (which can be sourced from lunar atoms) like batteries.

Don't use lithium batteries to continuously power an oxygen generator. Instead, have an enlarged oxygen generator and store half the flow for use at night.

Don't use lithium batteries to power a heater at night, instead "pre-charge" heat in a dirt thermal mass during the day.

Don't use lithium batteries to power a continuous CO2 scrubber, instead use a liquid amine scrubber and recharge the fluid tanks during the day.

Etc etc

Obviously you need to do in-depth tradeoff math for each case, but my napkins say "the system is the battery" wins in many cases.

Lunar night being about 14 days might make that a bit more difficult for any permanent settlement.

Storing 14+ days worth of <everything> is going to mean massive amounts of storage.

Yup. Nuclear seems likely to be part of the solution.

>Storing 14+ days... is... massive

...which is why you want to be extra careful in how mass-efficient you are storing it.

A tank of liquid oxygen masses only a few percent of its contents, and its contents store more chemical energy per mass than a lithium battery. Later on, insulated tanks are vastly easier to produce from lunar materials vs batteries. These are all nice leverage multipliers.

MEA amine solution is made of 80% lunar-abundant elements, and stores 22% its mass in CO2.[0] Importing the amine, tanks, and extra equipment requires under 3 kg/person for energy storage, vs 34 kWh[1] of batteries.

Nuclear is a popular notion, but the real-world economics don't seem to favor it. Effectiveness beats elegance,[2] apparently.

[0] https://en.wikipedia.org/wiki/Amine_gas_treating

[1] https://www.nasa.gov/sites/default/files/files/Gellett_Solid...

[2] https://spacecraft.ssl.umd.edu/akins_laws.html

> A tank of liquid oxygen masses only a few percent of its contents, and its contents store more chemical energy per mass than a lithium battery.

Making liquid oxygen (as opposed to gaseous oxygen) is not a terribly efficient process.

Naturally, there's a trade between LOX and pressurized oxygen gas. Reused propellant tanks of one Starship at 6 bar would hold ~14 metric tons of oxygen, enabling a continuous lunar population of over 1,000 people.

The energy for compression or liquefaction isn't necessarily a show-stopper, it just works into the total energy efficiency of the storage system. Compression is nice because practical isothermal compressors are now being demonstrated.[0]

Both liquefaction and compression require cooling. I do think it probably makes sense to do the thermal storage trick in reverse also, storing up a bank of "cold" at night to be used during the day. The thermal radiator panels still provide (derated) cooling in the day, but the thermal storage pool time-shifts the natural oversupply of cooling power at night.

[0] https://newatlas.com/good-thinking/carnot-centrifugal-air-co...

Is setting up shop at the poles feasible?

I suspect there are some great solutions to power storage on the moon, eg why not store power in molten metals/substances? The lack of an atmosphere would help massively reduce thermal loss, and as you already have a refinery much of the infrastructure for this already exists.

> The lack of an atmosphere would help massively reduce thermal loss

Care to elaborate? As an interested layperson I always thought the major factor for losses in open air was radiation into space. As such an atmosphere's thermal mass would be benfitial, no?

> The bigger problem is how practical is it to use PV at all on much of the moon.

A tactic I've used in Dyson Sphere Program is to build an equatorial belt around the entire moon so that some % of panels are always in sunlight. On our actual moon that would be approximately 7.2m panels, assuming they were a single tightly packed row of 5' x 3' panels.

This sounds like an insane amount but several of the largest solar installations on Earth each have more than that, with Bhadla Solar Park in India having over 10 million[1]. It's a dumb solution but is one which requires no shipment of energy storage. If you're able to mass manufacture panels in-situ, it may be simpler to do something like that than to ship large batteries to the Moon in order to use them as storage.

[1]: https://www.aljazeera.com/gallery/2021/11/3/india-solar-rene...

An alternative path is that there are permanently illuminated areas near the "north" pole of the moon. This has a somewhat lower barrier to entry.

They claim to achieve 99.999% purity on the silicon, so it sounds like they might do better than 6-9% on efficiency. But then again if they were anywhere near Earth produced panels, they would probably be saying that to everyone who would listen.

Nine nines is the usual target for semiconductor work… five nines is off-the-shelf. It’s impressive for “on the moon,” but I’d expect a significant performance shortfall from what we’re used to.

Surprisingly it appears not to be too far off standard solar panel efficiencies. According to this source[0], five nines silicon (5N) is called Upgraded Mettalurgical-grade (UMG) silicon. According to this paper[1], efficiencies over 20% have been reached with UMG silicon.

[0]http://www.greenrhinoenergy.com/solar/technologies/pv_manufa... [1]https://www.sciencedirect.com/science/article/abs/pii/S00380....

I mean, how many of these do you need to produce as a seed? You certainly don't start the process at full capacity; if there are ways this nests into an evolving, self-reinforcing system to prepare for human contact then it's a fantastic start.

It's not like we have huge space constraints for small starts, I reckon?

99.999% is what we used in our 156.25mm x 156.25mm polycrystalline cells when I did solar manufacturing. 21% efficiency was easily obtained, allowing ~300w in a 60-cell panel.

However, there's no atmosphere or weather to worry about on the moon, so I'd expect that to improve efficiency quite a bit compared to the Earth.

The real challenge with solar power on the Moon remains the two-week day/night cycle, which means either massive batteries (impractical) or awkward polar cliff locations to minimize the hit.

Or a (partial) ring around the equator, video game style.

There is a thin "atmospheric" layer of molecules floating around the lunar surface. Indistinguishable from vacuum for most purposes but it might affect that many 9's of purity.

https://www.nasa.gov/mission_pages/LADEE/news/lunar-atmosphe...

> Another big question is how much mass needs to be shipped up from earth to manufacturer solar cells.

If you take the linked page at face value, it explicitly states:

> without special substances brought from Earth.

(I suspect, like most press releases, some important details aren't being provided. Maybe dopants aren't considered "special substances")

This study[0] estimated that less than a microgram of dopant is needed per m^2 and that producing 5 MW of solar cells requires 1 gram of phosphorous. [0]https://ascelibrary.org/doi/abs/10.1061/(ASCE)0893-1321(2001...

Isn’t solar exposure on the moon stronger than earth due to the lack of an atmosphere? Efficiency per sqm might end up being higher.

I think the implied comparison was: do we launch panels to the moon or build them in-situ? The increased solar exposure should boost both of these cases by the same percentage, but if the Earth-manufactured panel is (e.g.) 3x as efficient, it might still make more sense to ship them.

Yep, you get about 36% more sun above the atmosphere. 1361 W/m^2 above the atmosphere vs ~1000 W/m^2 on a clear day [0].

Now you just need to bring big enough batteries to survive 14 days in a row with no sun.

[0] https://en.wikipedia.org/wiki/Solar_irradiance#Irradiance_on...

Considering the fact that you have the entire moon as available surface, you'd rather lift a giant rock with that power and store all the potential gravitational energy as a "battery".

A 10 ton rock, lifted up to 50m high, will store up to 810M Joules at moon gravity

>A 10 ton rock, lifted up to 50m high, will store up to 810M Joules at moon gravity

Wrong prefix? I think you meant 810K Joules.

For Earth gravity: 10,000 kg times 9.8 m/s^2 times 50 m = 4.9 megajoules, 1.36 kilowatthours. (Anker sells a 2.04 KWh battery pack for US$1999: https://www.anker.com/products/a1780 The manual says it masses 30.5 kg, so that's a cool 491.88x difference in mass power density.)

Again, that's for Earth gravity. Lunar gravity: 10,000 kg times 1.62 m/s^2 times 50 m = 0.81 megajoules, 0.225 kilowatthours. (2,976x difference in mass power density vs the lithium battery.)

Gravity storage isn't economical on Earth, and it's really not economical on the Moon, where gravity is lighter.

Unfortunately I think you're off by an order of magnitude. I think it would be 810 Kilojoules, which is approximately equivalent to a 1kg lithium-ion battery. Of course, you could move thousands of rocks up and down a big crater, rather than just one, but it would still be a lot of infrastructure for a fairly small amount of energy storage.

Three orders of magnitude*

No need for reliance on batteries. Just use the solar generated to process regolith yielding plenty of hydrogen and oxygen for energy storage, breathing, rocket fuel and additional component elements.

I think you might find places relatively close each other in the moon polar areas that are in aggregate illuminated most if not all the time.

https://space.stackexchange.com/questions/34025/moon-polar-d...

Wouldn't it be smarter to build on the far side of the moon? Then you don't need to worry about the Earth obscuring the sun at all, right? Or were you referring to some other phenomena?

You still get the Moon obscuring itself. The Earth is not eclipsing the Sun most of the time.

the moon is tidally locked to the Earth, i.e. it does not rotate about an axis, and takes ~27 days to orbit. therefore around half of those days are in darkness at any given point on its surface, hence the lunar phases

Ah yeah, makes sense... I confused myself.

The alternative is that you could also just plan around the lunar cycle by turning machinery off during the lunar night and only having moon landings during the day.

You still need heaters, because a lot of equipment -- including solar panels -- won't survive thermal cycling otherwise.

There are a bunch of things that are better about the Moon: - no atmosphere in the way - no weather or clouds in the way - no buildings or other stuff competing for the land area (yet)

There's no shortage of places to put solar panels, and nothing that will reduce the amount of light getting to them (except the rotation of the Moon itself), so actual efficiency isn't that important.

The Earth's atmosphere absorbs and reflects solar energy - only 48% of the solar energy reaches the surface of the planet[1]. I didn't see any mention of this on the link you provided, I don't know if that 6-9% figure includes this.

If it's not included, then the extra energy available could raise the output to an equivalent of a 12-18% efficiency cell on Earth's surface, wouldn't it?

[1] https://earthobservatory.nasa.gov/features/EnergyBalance/pag...